A magnetic random access memory (MRAM) uses, as a memory element, an MTJ (Magnetic Tunnel Junction) element using the magnetoresistive effect by which a resistance value changes in accordance with the direction of magnetization. The MTJ element has a three-layered structure including a reference layer, a memory layer, and an insulating layer that is sandwiched between the reference layer and memory layer and forms a tunnel barrier. The magnetization in the reference layer is fixed in one direction and does not reverse even when a write operation is performed. On the other hand, the magnetization in the memory layer reverses due to torque externally given by a write operation.
An MRAM using a spin-transfer torque writing method of writing data by directly supplying a current to the MTJ element is known. When a write current is supplied to the MTJ element, the resistance value of the MTJ element changes depending on the relative directions of magnetization in the two magnetic layers. That is, the resistance value of the MTJ element becomes low when the magnetization directions in the memory layer and reference layer are parallel, and becomes high when the magnetization directions are antiparallel. The MTJ element can be used as a memory element by making these low- and high-resistance states of the MTJ element correspond to binary data.
Generally, a magnetic layer having magnetic anisotropic energy higher than that of the memory layer is used as the reference layer, so a leakage magnetic field from the reference layer is large. Therefore, the leakage magnetic field from the reference layer acts on the memory layer, and the magnetic coercive force of the memory layer shifts. Consequently, a current for switching the magnetization in the memory layer increases, or the thermal stability of the MTJ element decreases. Also, as the micropatterning of the MTJ element advances, the leakage magnetic field from the reference layer increases. As a consequence, the magnetic coercive force of the memory layer largely shifts.